The present invention relates to an optical head adapted for use in an apparatus, which can record information on an erasable recording medium, retrieve and erase the recorded information from the medium, and more specifically, to an optical head of a simple construction for the apparatus.
Conventionally there have been developed apparatuses which can optically record on, or retrieve or erase information from an information storage medium, using magnetooptical effects, such as the Kerr effect, Faraday effect, etc. These apparatuses generally use an optical head, as shown in FIG. 1 or 2.
In the optical head shown in FIG. 1, an elliptic light beam, emitted from semiconductor laser 10, is transformed into a circular, parallel beam, by collimator lens 12 and ellipse compensating prism 14. The collimated beam is led to objective lens 20, through beam splitter 16 and mirror 18, and is converged on information storage medium 22.
The light beam, reflected by information storage medium 22, is reflected again by mirror 20 and beam splitter 16, and then split into two divergent beams by beam splitter 24. One of the divergent beams is transmitted through half-wave plate 26, to have its polarization face rotated through approximately 45 degrees, and is then converged by convex lens 28. The converged beam is split into P- and S-polarized light components by polarized-beam splitter 30. Light beams equivalent to the P-polarized and S-polarized components are incident on photodetectors 32 and 34. The other divergent beam from splitter 24 is transmitted through convex lens 36 and cylindrical lens 38, to be incident on photodetector 40.
In the optical head thus constructed, a focusing error signal is delivered from photodetector 40, while a tracking signal is delivered from one of photodetectors 32 and 34. Information stored in information storage medium 22 is read out in accordance with the difference in intensity between the P- and S-polarized component beams, i.e., the difference between output signals from photodetectors 32 and 34.
The optical head shown in FIG. 2 has no beam splitters, but is provided with spherical concave lens 42 instead. Photodetector 40, which is used to detect a P-polarized component light beam from polarized-beam splitter 30, produces a focusing error signal. Photodetector 44, which serves to detect an S-polarized component light beam from splitter 30, produces a tracking signal. The information stored in information storage medium 22 is read out in accordance with the difference between output signals from photodetectors 40 and 42.
In these prior art optical heads, the so-called differential detection method is used in reading the information from the information storage medium, in order to improve the signal-to-noise ratio of retrieved information signals. The optical head of FIG. 1 is provided with a focusing-error detection system, besides a retrieval signal detection system. Thus, it is complicated, requiring an increased number of optical components.
In the focusing error detection, the optical heads of FIGS. 1 and 2 both use the so-called astigmatism process (stated in Japanese Patent Publication No. 53-39123). In this process, the focusing error signal is liable to produce noise, influenced by a diffraction pattern, which is caused when a beam spot on the information storage medium crosses a continuous groove, used as a track guide. Thus, the focusing error sometimes cannot be detected accurately. If the storage medium is tilted, beam spots move on the photodetectors, so that the focusing error detection cannot be accurate.